Japanese Patent Publication No. 3448122 describes a guide of the type to which this invention relates. More particularly, it describes a guide in the form of a tensioner lever composed of a polyamide resin shoe attached to a support base composed of die cast aluminum or glass fiber-reinforced polyamide resin.
In internal combustion engine design, in order to achieve increased engine power output or torque, the engine displacement is often increased by increasing the piston and cylinder diameters, or by increasing the piston stroke. However, the external dimensions of the engine often remain unchanged. Thus, although the load imparted to the tensioner lever is increased as a result of the increase in the engine displacement, there is no concomitant increase in the size or strength of the tensioner lever. The useful life of the tensioner lever is therefore shortened.
The strength of a tensioner lever can be increased by increasing the thickness of its shoe support base. However, in most cases, because of limitations in the space available for the tensioner lever and adjacent engine parts, it is difficult to increase the thickness of the lever's shoe support base.
The strength of the shoe support base can be increased, without changing its size and shape, by forming the base from a different material such as a high strength steel. However the adoption of a higher strength material for the shoe support base results in higher manufacturing costs both because of the increased cost of material, and because of the increased difficulty of molding the higher strength material.
It is possible to address the problem of achieving adequate strength in a tensioner lever for an increased displacement engine, without changing the shape of the shoe support base, by molding the shoe from a high strength resin. Such an approach is illustrated in FIG. 6. The external shape and dimensions of the shoe support base 520 remain unchanged. However, the cost of the tensioner lever 500 is increased because of the high cost of the material of the shoe 510. Moreover, the hooks provided on the shoe for connecting the shoe to the shoe support 520 become less flexible with the adoption of the higher strength shoe material. The side walls, which extend outward from the chain-contacting surface of the shoe to limit lateral movement of the chain, also become less flexible, and impair the overall flexibility of the shoe. Consequently, assembly of the tensioner lever becomes more difficult, and cracks, splits, fissures and the like can form in the shoe, making failure due to shoe breakage more likely.
An object of the invention is to provide a guide which exhibits adequate strength for use with an engine having increased displacement, without changing its external shape or dimensions, without increasing its weight and production cost, without making assembly more difficult, and without impairing the longevity of the guide.
The guide in accordance with the invention comprises a shoe-supporting base adapted for mounting on an engine block and an elongated shoe having a sliding contact surface for sliding engagement with an endless, traveling transmission chain. The shoe is connected to, and supported by, the shoe-supporting base, and has at least one side wall extending from its sliding contact surface in a direction such that it can engage a side of a chain traveling on the sliding contact surface and thereby limit snaking travel of the chain. The shoe also has at least one hook securing the shoe to the shoe-supporting base. The guide is characterized by the fact that the shoe is a sandwich-molded resin structure, having an internal core and a skin layer, the internal core being composed of a resin having a higher strength than the resin of the skin layer, and by the fact that the side wall and hook are both unitary with the skin layer.
Preferably, the skin layer is composed of a polyamide resin, and the core is composed of a glass fiber-reinforced polyamide resin.
The invention makes it possible for a high strength shoe to be molded reliably with highly accurate outer dimensions. A guide incorporating the shoe exhibits high strength over a long period of time, and can used in an engine having increased displacement, without changing its shape or dimensions, and without increasing its weight or significantly increasing its manufacturing cost. Furthermore, the side wall (or side walls) of the shoe, and the hooks which connect the shoe to the supporting base, all of which are flexed temporarily when the shoe is incorporated onto the supporting base, remain highly flexible, and therefore allow the shoe to be incorporated onto the supporting base without difficulty.
When glass fiber-reinforced polyamide resin is used as the core of the shoe, the strength of the guide is remarkably improved, and the shoe is able to withstand the high loads imposed as a result of increased engine displacement. The polyamide resin forming the skin layer envelops the glass fiber-reinforced core completely, ensures that the guide will retain its shape, and exposes a surface on which the timing chain can slide with low friction, and with minimal wear.